The fabrication of advanced integrated circuits requires the formation of extremely small, precise features on a semiconductor wafer. Such features are typically formed first by a photolithography process in temporary layers of photoresist, and the photoresist features are then used to create permanent structures on the wafer. For example, holes are formed in insulating layers and later filled with a conductive material to create connections between layers in a circuit. Trenches are also formed in insulating layers and later filled with a conductive material to form capacitors. Groups of thin conductive lines are formed to make buses to carry signals from one area of a chip to another. The groups of conductors are characterized by the width of each conductor and a pitch, that is, the distance between the conductors.
As the precision requirements for semiconductor processing increases, there is a need to constantly monitor the fabrication process to ensure that it is meeting the stringent requirements. In some cases, every wafer going through the fabrication line is measured in what is sometimes referred to as in-line metrology. Engineers may monitor both the features on the temporary photoresist layer and the permanent features created on the wafer. Features on the wafer are three-dimensional structures and a complete characterization must describe not just a surface dimension, such as the top width of a hole or conductor, but a complete three-dimensional profile of the feature. For example, although an ideal feature typically has vertical sidewalls, the actual sidewalls may have excessive slope that narrows or widens the feature below its top surface. Process engineers must be able to accurately measure the profiles of such surface features to fine tune the fabrication process and assure a desired device geometry is obtained.
During process development, one method of characterizing the fabrication process results is by sectioning, that is, cutting a wafer through the feature to be characterized, and then observing the exposed cross section using a scanning electron microscope (SEM). While useful in developing new processes, this method is less useful in monitoring production processes because the entire wafer is destroyed to measure a feature. Moreover, it is time consuming to section the wafer at the correct position and view it in an SEM.
Three processes that are often used for such in-line metrology are critical dimension scanning electron microscopy (CD SEM), scanning probe microscopy (SPM), and scatterometry. CD SEM entails using a scanning electron microscope to create an image of the top surface of the wafer being processed. CD SEM is particularly useful in monitoring the critical dimensions, such as the top width of a hole or trench or the width of conductors. Because it displays a top view, CD SEM does not usually provide any information on the three-dimensional profile of such holes or conductors, and may not alert a process engineer if the sidewalls of a feature are deviated from the vertical.
Insulators and photoresist tend to become electrically charged by the electrons in the CD SEM, and this charging causes the edges of the feature image to blur, making measurements uncertain by between 2 nm and 25 nm. The edge blurring effect can be characterized by measuring features having known dimensions and subsequent CD SEM measurements can be partially compensated to correct for the edge blurring. However, the charging is dependent upon the composition and thickness of a number of the layers underlying the feature. If the device design changes or even if the process for the underlying layers drifts, a separate set of calibrations is required.
A further drawback with electron microscopy is that the measurement needs to be performed in a high vacuum to prevent the probing electrons from being scattered by air molecules. It takes considerable time to remove the air from an SEM sample chamber, thereby preventing rapid feedback and limiting the number of wafers that can be measured.
The second method of in-process measurements, SPM, uses a very small probe tip that is scanned across the wafer surface. There are many types of SPMs, including scanning tunneling microscopes and several types of atomic force microscopes (AFM). In one type of AFM used in semiconductor processing, the probe tip is moved vertically and horizontally into contact or near contact with the surface. The vertical positions at or near contact are tabulated and provide a profile of the surface. AFMs have been demonstrated to achieve resolution of the order of 1 nm, which is adequate for most advanced processes, and they can be operated at atmospheric pressure.
However, AFMs suffer from low throughput. AFMs require not only horizontal scanning, but also some type of vertical scanning. The vertical scanning can be substantially reduced by a feedback control of an oscillatory vibration of the tip operated in the non-contact mode, but sharply profiled features reduce this advantage. It is difficult to initially align the probe tip with nanometer-size feature. As a result, except when measuring test patterns in the shape of gratings, a large number of parallel scans must be performed to assure that the probe encounters the feature. It can take several minutes to make measurements across one feature and several hours to measure a 50 μm square area. For these reasons, AFMs in production operation can profile only a limited area of the chip.
The third type of in-process inspection methods, generally referred to as scatterometry, entails directing light onto a test pattern on a surface and measuring the reflected light. The reflected light is affected by the geometry and composition of the target. For example, the results are affected by the width and spacing of repetitive features, the composition of the material at and below the surface, including the thickness of any layers near the surface. The result of a scatterometer measurement is typically a “signature” or graph, showing the variation in the intensity of the reflected light as the wavelength or the angle of incidence changes.
Because scatterometry requires a regular grid to create the diffractive effect measured, when scatterometry is used to determine dimensions, it is typically used on isolated test patterns, rather than on the circuit itself. The test patterns are created on unused portions of the wafers, typically between the individual integrated circuits, at the same time that the actual circuits are created so that the test patterns reflect the processes that are creating the actual production product, that is, the integrated circuits. The test patterns typically consist of a grating pattern about 50 μm×50 μm. Scatterometry uses a relatively wide beam of light and provides average information about the geometry over the area of the beam. Thus, rather than determining the width of a particular line or feature, scatterometry determines, for example, the average line width over the smaller of the spot size of the optical assembly or the test pattern. An optical instrument, such as a scatterometer, that integrates information from multiple features to produce a result is referred to as an integrating optical instrument, as opposed to an individual-feature-measuring instrument, such as an AFM, SEM, or focused ion beam system, that measured a characteristic of a single feature.
Typically, scatterometry is practiced with an ellipsometer or a reflectometer in which a probing beam of radiation having a diameter of about 25 μm to about 200 μm strikes the test grating pattern at a fixed angle with respect to the surface normal and to the grating structure axes. A polarization-sensitive optical detection system is arranged to detect the radiation reflected from the surface. There are two general approaches to the types of ellipsometric data used for scatterometry. In a first approach, referred to as spectroscopic scatterometry, the optical detector is set to detect a beam reflected at a complementary angle about the surface normal, that is, a first-order reflection, and the data is obtained over a range of wavelengths of incident light. The acquired data may simply be the spectrally resolved intensity, or it may be the dual sets of data possible in ellipsometry, for example, the spectrally resolved Ψ and Δ parameters well known in ellipsometry. In a second approach, referred to angle-resolved scatterometry, a single wavelength is used, but the detector is scanned over a range of angles. In either case, the ellipsometer produces at least one distinctive trace over either wavelength or angle.
Scatterometry is well suited to process control. It non-destructively probes the wafer so that the wafer can be returned to the production line, and it can be performed at atmospheric pressure with equipment occupying relatively little space. A complete set of scatterometry data can be acquired from a test site in about 200 ms. Moving between test sites on a wafer can be done in less than 3 seconds. Since a typical process sequence in IC fabrication has a throughput of less than one wafer per minute, scatterometry can probe many test sites on every wafer without interrupting the process flow.
The general process for using scatterometry for process control is described by Allgair et al. in “Manufacturing Considerations For Implementation Of Scatterometry For Process Monitoring,” Proc. of the SPIE Conference on Metrology, Inspection, and Process Control for Microlithography XIV, Feb. 28–Mar. 2, 2000. It is generally acknowledged that for multi-layer structures, the ellipsometric data cannot be reasonably and directly interpreted to determine the grating structure, or even just the grating pitch, line width, or line spacing ratio. Instead, libraries of data are generated by performing optical scattering calculations for structures in which only some of the parameters are varied and the others are assumed known. For example, one may assume a particular pitch, straight vertical walls, and a particular material composition and layer thickness, and then calculate a set of patterns or graphs based upon different line width. Then, during an actual measurement, the scatterometry graph is compared to the graphs in the library. Although the library information is referred to graphs or patterns, it is understood that the process of generating measurement data and comparing the measurement data to the library data can be performed entirely in software, without producing any actual images of graphs or patterns. The line width parameter of the library graph closest to the experimental data is assumed to be the line width of the measured structure. The closeness may be determined by calculating the root mean square error between each library graph and the experimental data and choosing the library graph with the smallest root mean square error.
This approach, however, assumes that attributes of the test structure that are not being measured, for example, the slope of the vertical walls, the material composition, and the layer thickness, are the same as the attributes assumed in the generation of the library patterns. If some attribute is different, the results are inaccurate and a new library must be calculated. It is generally considered infeasible to consider all possible structures in seeking the structure with the minimum root mean square error. Therefore, scatterometry is limited by the need to assume certain characteristics of the structure to be measured.
In general, scatterometry is good at detecting small changes and variations in processing but ill suited for larger changes because the results of larger changes cannot be matched with library patterns. Although SEMs and atomic force microscopy are effective at detecting significant changes, they are less suited to a production environment. Furthermore, they are relatively insensitive to compositional variations that are important for device reliability.